1. Field of the Invention
This invention relates generally to drill bits and drill bit cutter arrangements primarily for use in plastic formations, and more particularly relates to a bit that includes cooperative combinations of positive and neutral or negative rake cutters.
2. State of the Art
Conventional rotary drill bits typically employ hardened cutters formed of materials such as polycrystalline diamond compacts (PDC's), boron nitride, or tungsten carbide and disposed on the bit face in order to produce shearing forces in the formation to be cut. Ordinarily, these cutters are angularly positioned on the face of the drill bit according to the formation material that they are designed to cut.
For example, positive rake or "front raked" cutters have an angle of inclination in the direction of bit rotation of greater than 90.degree.. In other words, positive rake cutters lean forward, or in the direction of bit rotation, and the included angle between the cutter face and the formation in front of it is greater than 90.degree.. These positive rake cutters tend to "dig in" to the formation material, as they do not put additional compressional stresses into the formation, which would give it a higher effective strength. The rotation and weight on the drill bit encourages these positive rake cutters to cut into the formation to their fully exposed depth, which could risk stalling of the bit. However, the hardness of the formation material may resist full depth penetration by the positive rake cutter. Thus, in relatively hard material the positive rake cutters will typically not invade the formation material to their full depth, although the possibility of stalling the drill bit may still be a consideration.
On the other hand, a drill bit having positive rake cutters that is used in a formation having a greater plasticity will likely result in full depth entry of the positive cutters and will correspondingly result in high torque which may stall the bit. Accordingly, drill bits designed primarily for use in formations of greater plasticity typically employ cutters having a negative rake.
The face of a negative rake or "back raked" cutter has an angle of inclination or included angle relative to the formation, that is less than 90.degree., or opposite to that of a positive rake cutter. In use, the negative rake cutter has a tendency to "ride" along the surface of the formation giving it a higher effective strength and more "plasticity," resisting entry into the formation and making only a shallow cut as a result of the weight on the bit. It can be seen that while negative rake cutters advantageously resist stalling of the bit in plastic formations because of lower aggressiveness, the linear rate of cut for a bit having negative rake cutters is typically substantially less than the linear rate of cut for a bit having positive rake cutters.
It is known in the art from U.S. Pat. No. 4,554,986 to utilize positive rake cutters disposed on a radially-oriented ridge on a bit face, trailing and separated from a leading radially-oriented ridge, the former being devoid of cutters but having wear elements embedded therein. The leading ridge limits the depth of penetration of the positive rake cutters on the trailing edge.
It is also known in the art from U.S. Pat. No. 4,981,184 to utilize ridge-mounted positive rake cutters disposed on a bit face in trailing relationship to ridge-mounted, dome-shaped "cutter inserts" which purportedly deform and stress the formation being drilled to its elastic limit, following which the positive rake cutters clip off the deformed formation. Each positive rake cutter is preceded by a dome-shaped cutter insert.
The cutter penetration limitation approach as described in the '986 patent does not take advantage of the cutting characteristics of positive rake cutters. The '184 patent, on the other hand, seeks to employ a "twin blade" approach similar to that utilized with facial razors, and is admirable in theory. However, variations in formation characteristics, pressures, drilling fluid weights and compositions during actual drilling all serve to preclude the realization of an actual drill bit performing in the manner described.